$7st + 9su + 2s + 6 = 4t - 9$ Solve for $s$.
Combine constant terms on the right. $7st + 9su + 2s + {6} = 4t - {9}$ $7st + 9su + 2s = 4t - {15}$ Notice that all the terms on the left-hand side of the equation have $s$ in them. $7{s}t + 9{s}u + 2{s} = 4t - 15$ Factor out the $s$ ${s} \cdot \left( 7t + 9u + 2 \right) = 4t - 15$ Isolate the $s$ $s \cdot \left( {7t + 9u + 2} \right) = 4t - 15$ $s = \dfrac{ 4t - 15 }{ {7t + 9u + 2} }$